Peak-to-average power reduction method

ABSTRACT

A method of reducing peak-to-average power in a hybrid signal is provided. The method determines peaks in power by defining a sample point by way of a digital vector and an analog vector. The digital and analog vectors are added together to generate a hybrid vector which is used to compare the sample point to the maximum desired peak threshold. An error vector is used to correct the sample point to a desired power level. Once the sample point has been corrected it can be added back to the analog signal and transmitted.

FIELD OF THE INVENTION

The present invention relates generally to a method for reducingpeak-to-average power in a hybrid signal comprising first and secondseparately modulated signals. More specifically, the invention relatesto a peak-to-average power method for use on a hybrid digital/analogradio frequency signal.

BACKGROUND OF THE INVENTION

In the United States, the In-Band On-Channel (IBOC) technology, referredto as HD Radio™, has been selected by the Federal CommunicationCommission to be the standard for simulcast digital programming alongwith traditional analog audio at the same frequency band. In otherjurisdictions, other standards have been adopted. For example, severalEuropean Union countries have implemented Digital Audio Broadcasting(DAB) for FM broadcasts and Digital Radio Mondiale (DRM) for AMbroadcasts.

HD Radio™ IBOC technology is proprietary to iBiquity DigitalCorporation, which develops and licenses the various components andtechnologies required for HD Radio™. As such, the instructions providedwith the equipment and technology, and training provided by iBiquityforms the common general knowledge in this field and allows thetechnology to be put into practice.

IBOC makes use of orthogonal frequency division multiplexing (OFDM)signalling. While it can be shown that OFDM provides substantialbenefits in digital wireless communications, one of its maindisadvantages is the fact that in the time domain the multitude ofsubcarriers add constructively or destructively almost at random. Thisproduces a time domain signal with widely varying power.

A high level combined broadcast transmission system takes the outputfrom a full power analog transmitter and combines it with the signalfrom a linear digital transmitter before sending it to the antenna. Inthis case, the digital power amplifier must be able to handle the fullrange of the digital signal's power fluctuation. In order to make IBOCdeployments economically feasible, peak-to-average power (PAPR)algorithms have been developed to reduce the power peaks in the digitalsignal. For example, U.S. Pat. No. 6,128,350 to Shastri et al., and USPatent Publication No. 2005/0169411 to Kroeger both describe PAPRalgorithms for use on the digital signal in IBOC systems. These standardalgorithms effectively brings the original 12 dB PAPR under 8 dB bysolely operating on the digital signal.

Conversely, a low level combined broadcast transmission system consistsof a single combined power amplification chain. In this case the analogand digital signals are added as digital complex baseband signals. Theaddition of the analog signal to the digital signal generates a hybridsignal and alters the power characteristics of the combined signal.

FIG. 1A-C illustrates a complex plane, where the X-axis reflects thebaseband signal's real (or in phase-I) component and the Y-axisrepresents the signal's imaginary (or quadrature-Q) component. As shownin FIG. 1A, the output of the FM modulation process produces a constantenvelope signal with varying phase. At baseband, this signal isrepresented as a vector 120 in the complex plane with constantamplitude, which is represented by a circle 107. Any given sample pointcan be represented by an additive vector 110, which represents thepossible amplitude and phase of the additive digital signal.

In theory, as shown in FIG. 1B, the standard PAPR reduction schemecreates a circle 140 having a radius that is equal to or less than thedistance between the constant FM signal level and the maximum desiredpeak threshold 112. Only sample points that fall within this circle 140can be certain not to add to the analog signal 102 constructively, andthus do not require correction. However, all sample points that falloutside of the circle 140 will require correction to a point within thecircle 140. For example, sample points 150, 151 and 152, defined bydigital vectors 110A, 110B and 110C, respectively, will all requirecorrection to be within the confines of circle 140.

However, this form of peak detection results in the digital signal beingunnecessary corrected in a variety of circumstances and largecorrections being applied when much less correction is required. Forexample, as shown in FIG. 1C, sample point 152 defined by digital vector110B would be corrected by correction vector 160, when in fact nocorrection would be necessary, since the actual sample point 152 wouldfall below the maximum desired threshold 112.

Sample point 150, defined by digital vector 110C, illustrates thesituation where a large correction would be applied to a sample pointrequiring only a small correction. In this example, digital vector 110Cprojects to a point 150 that is just beyond the maximum desired peakthreshold 112. In the standard PAPR reduction scheme, a large correction161 would be applied to this vector 110C to bring the sample point intothe circle 140, whereas in reality a small correction function couldhave been used to bring the sample point 150 below the maximum desiredpeak threshold 112. Only in circumstances where the digital vector couldadd to the analog vector in-phase to generate a maximum peak does thecurrent PAPR reduction scheme apply proper clipping or correction. Forexample, sample point 151 defined by digital vector 110A generates amaximum peak, in which a proper correction vector 162 is applied.

Accordingly, there is a need to develop a method that does notunnecessarily reduce peaks in power that fall within the maximum desiredpeak threshold and therefore overcomes the limitations of the prior art.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a peak detectionmethod that overcomes the limitations of the prior art and is applied toa hybrid signal.

It is a further object of the present invention to provide a method forpeak detection that takes into account the contribution of eachseparately modulated signal and uses this information in reducing thePAPR in the hybrid signal.

According to an aspect of the present invention there is provided amethod for peak-to-average power reduction in a hybrid signal comprisingfirst and second separately modulated signals, the method comprising thesteps of: obtaining a sample point of the hybrid signal; defining thesample point by way of a first vector corresponding to the firstseparately modulated signal and a second vector corresponding to thesecond separately modulated signal; adding the first vector to thesecond vector to obtain a hybrid vector; comparing the hybrid vectoragainst a maximum desired peak threshold to identify peaks in power; andadding an error vector to the second vector to reduce the peak.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription and accompanying drawings wherein:

FIG. 1A is a complex plane, where the X-axis reflects the in phase (I)component and the Y-axis represents the signal's quadrature (Q)component;

FIG. 1B is a complex plane depicting the addition of a digital vector toan analog vector in accordance with the prior art;

FIG. 1C is a complex plane depicting correction of the digital vectorsin accordance with the prior art;

FIG. 2 is a block diagram of the peak-to-average power scheme of thepresent invention;

FIG. 3A is a complex plane depicting the addition of a digital vector toan analog vector in accordance with an embodiment of the presentinvention; and

FIG. 3B is a complex plane depicting the correction of the hybrid vectorin accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of one particular embodiment by way ofexample only and without limitation to the combination of featuresnecessary for carrying the invention into effect.

As a representative example, description of the method is given usinghybrid FM IBOC broadcast transmission. However, persons of ordinaryskill in the art will readily understand that the general method,software and system can be applied to other hybrid transmission schemesthat add two separately modulated signals, such as an OFDM signal and anexisting transmission standard.

It should be also noted that the present invention can be carried out asa method, can be embodied in a system, a computer readable medium or anelectrical or electro-magnetical signal.

As shown in FIG. 2, in a standard low level combined broadcasttransmission system, the digital and analog signals 101, 102 of the FMIBOC broadcast transmission converge to form a hybrid signal. Eachsignal 101 and 102 is separately modulated prior to convergence. In thepresent example, the digital signal 101 is modulated in accordance withan IBOC modulation scheme and the analog signal is modulated using astandard frequency modulation scheme. Peaks are detected, as shown byblock 190, by taking sample points within the hybrid signal andcomparing them against the maximum desired peak threshold. Any peakdetected is then reduced, as shown by block 191, to below the maximumdesired peak threshold. From this point the signal may be furthercorrected by spectrum and constellation correction 192 or passed forwardwithout further correction to the next step in the broadcast stream.Several iterations of the method can take place before the digital andanalog signals 101, 102 are recombined, as shown by block 193, to beoutput as a hybrid signal (block 194).

In this low level combined broadcast system, it has been found that ifthe hybrid signal is also modulated, then a PAPR of around 3 dB can beachieved even with a increase in carrier levels of up to 10 dB.Comparatively, using the standard IBOC PAPR methods, as described inShastri et al., (U.S. Pat. No. 6,128,350) and Kroeger (US 2005/0169411),a transmitter capable of handling a PAPR of up to 4.5 dB will berequired to accommodate the 10 dB increase in digital carrier levels.This will result in a dramatic increase in transmitter overhead.

The standard PAPR reduction methods developed for IBOC broadcasttransmission, such as Shastri et al., (U.S. Pat. No. 6,128,350) andKroeger (US 2005/0169411), are sub-optimal for use on the hybrid signal100. This is in part due to the fact that applying the PAPR reductionalgorithms of Shastri et al., (U.S. Pat. No. 6,128,350) and Kroeger (US2005/0169411) on the hybrid signal 100 would obliterate the analogsignal 102 content. In order for PAPR reduction to take place on thehybrid signal 100 the sections of the signal that contribute to thedevelopment of a peak should be identified.

According to the present invention a signal peak, which is defined as asample point exceeding a maximum desired threshold, is determined byadding the analog vector to the digital vector. The resultant hybridvector is then thresholded against the maximum desired peak threshold.If the digital signal adds constructively to the analog signal, then alarge correction will be required. A smaller correction is needed, ifthe vector addition falls close to the maximum desired peak and nocorrection is required if the digital signal adds destructively to theanalog signal and resultant hybrid vector projects below the maximumdesired peak threshold.

As shown in FIG. 3A, addition of a digital vector 110 to the analogvector 109 generates a circular area 113 around the analog signal point170. The area of the circle 113 can be further divided into twosections, namely a no correction area 114 and a correction area 116.Within correction area 116, a gradient relating to the amount ofcorrection to be applied covers the area with a large amount correctionrequired near the section where the digital vector 110A would add to theanalog vector 109 in-phase. The amount of correction required wouldgradually drop off as the digital vector 110A moved away from themaximum addition situation. According to the present invention, only ifthe sample point falls within the corrections area 116 will a correctionbe applied to the signal. However, a majority of the area of the circledefined by the hybrid vector 111 will not require a correction, sincethe sample points will fall below the maximum desired peak threshold112. For example, as shown in FIG. 3B, sample point 180 falls below themaximum desired peak threshold 112. Since the sample point 180 isdefined by the hybrid vector 111A, no correction is required since thevector 111A projects outside the correction areas 115 and 116.Conversely, using the standard PAPR reduction scheme, the same samplepoint would be unnecessarily clipped to within the circle 140 (see FIG.1C).

If a sample point falls within the correction area 116, near the maximumdesired threshold 112, a lower amount of correction will be required tobring the signal below the maximum desired peak threshold 112. Forexample, sample point 181, shown in FIG. 3B is slightly higher than themaximum desired peak threshold 112. Since the sample point 181 isdefined by the hybrid vector 111B, only a slight correction is required,along the hybrid vector 111B, to bring clip the sample below the maximumdesired peak threshold 112. By introducing a lower amount of correction,the peak detection method of the present invention can achieve the samemaximum desired peak value with a lower degree of distortion in theoriginal signal. This allows the signal peak to be reduced furthercompared to the standard method of PAPR applied only to the digitalsignal 101.

In situations where the hybrid vector 111 projects essentially in thesame direction as the analog and digital vectors 109, 110, a largecorrection, essentially the same as with the standard PAPR reductionscheme, will be required.

Once the peaks have been detected, a reduction algorithm is applied toreduce the peaks to a point below the maximum desired peak threshold112. In typical PAPR reduction schemes, peaks are reduced by adding acorrection vector to the digital vector to thus reduce the magnitude ofthe sample point to below the maximum desired peak threshold (see FIG.1C). In the case where a hybrid vector is used to detect the signalpeaks, adding a correction vector to the hybrid vector will besub-optimal for the eventual constellation and spectrum correctionsteps. Accordingly, peak correction is achieved by adding an errorvector, based on the correction vector, to the digital vector componentof the hybrid vector.

In the method, one threshold is used to make a decision to correctionthe signal and another value determines the amount of correction to beapplied. This allows the peak to be over-clipped to ensure that it willnot come back to such a level during the correction phase.

The person skilled in the art will readily appreciate that the signalcan be corrected using any number of correction functions known in theart. For example, the correction function can be accomplished by eitherhard or soft clipping the digital signal portion of the hybrid signal.Hard or soft clipping adds a delta function defined by δ(t) e^(jΦ) tothe digital signal, where Φ represents the angle of the correctionvector.

In another embodiment, the correction function may be a real or complexfunction, such as a complex frequency. However, the correction functionshould present a real point at the peak to be corrected, so that thecorrection angle Φ can be added to the point.

It may also be possible to compute the correction vector in parallel butopposed to the analog vector so as to preserve the phase of the analogvector. This introduces asynchronous noise to the FM modulated signal,but would allow the frequency space of the analog signal to be employedfor peak correction.

A sinc ( ) function may be included in the generalized peak correctionformula to allow for the delta function to be applied to the integersample points, but can be used to shift the delta function to liebetween sample points, should the actual peak not fall on discretesample points. This can reduce the need to operate on an oversampledsignal and may prove to be computationally more effective.

In one embodiment, additional oversampling for the peak detection stepmay be beneficial and even may allow the overall oversampling factor tobe omitted. If a peak is detected in-between samples, the correctionfunction must be shifted accordingly. In the case of clipping using adelta function, the sinc( ) function of the correction function isshifted, which provides non-zero time domain values across a larger timewindow.

Based on the correction vector, an error signal (representing an errorfrom the original signal) can be generated by the following generalformula:

$\left| {E\lbrack n\rbrack} \right. = {\sum\limits_{K = 0}^{{length} - l}{{{correctionFunction}\mspace{14mu}\left\lbrack {n - k} \right\rbrack}{C\lbrack k\rbrack}}}$

The introduction of an error signal based on the formula identifiedabove, allows alternate means of peak correction, such as tone injectionor pulse injection. Furthermore, the correction function may even bedetermined adaptively and may not be the same or even static acrossiterations or individual peaks.

In order to limit the possibility of the peak re-emerging whilecorrecting the constellation and spectrum, the correction vector can beextended further than normal using the above formulas. Further, thecorrection vector by be computed along the original digital vector,rather than to the origin. This would allow the phase of the digitalsignal to be preserved.

The use of an error signal to correct the hybrid signal allows the shapeof the spectrum to be controlled, such that it requires less eventualconstellation and spectrum correction. Furthermore, additional frequencyspectrum may be used to more effectively reduce peaks. For example, ifthe extended carrier spectrum is not used for data carriers, it can beused to hold a larger portion of the error signal. Depending on thechoice of correction function, the error signal can be small enough thatthe constellation and spectrum correction step may be bypassedaltogether.

Modification of the digital signal in the time domain can negativelyimpact the signal constellation, as well as increase out-of-band noiserequiring correction. Unlike the standard PAPR reduction scheme,different correction functions in the proposed method will have varyingimpacts on the constellation and injected noise level. Some correctionfunctions, such as tone injection, may safely bypass the constellationand spectrum correction step.

The constellation and spectrum correction is applied to a modifiedversion of the digital signal. Alternatively, the combined error vector,without addition to the digital vector, may be tracked and a comparableconstellation and spectrum correction applied thereto. This would permitthe original digital signal to pass through the steps in an unmodifiedform until the error vector is finally added to the digital signal.Moreover, performing correction on the error vector rather than thedigital vector allows alternate means of constellation correction. Forexample, any of the QPSK constellation points could be rotated back in adesired boundary region rather than pushing points along the X and Yaxes.

The basic steps of constellation and spectrum correction involve:removing the pulse shaping of the IBOC signal; performing an FastFourier Transformation (FFT) operation to demodulate the OFDM carriers;applying a frequency mask that zeros or reduces frequency bins notassigned to IBOC carriers; constraining the constellation of IBOCcarrier points back toward the actual signal point up to a thresholdvalue along both the real and imaginary axes; correcting the phase ofall reference carriers; computing the Inverse Fast FourierTransformation (IFFT) to re-modulate the signal; and re-applying pulseshaping to the symbol.

The correction of the constellation points should be performed withrespect to the original Quadrature Phase Shift Keyed (QPSK) point.Basing the correction on the modified symbol itself can lead to biterrors, as the applied correction function could have moved theconstellation point into another decision region.

The process may operate at sample rates greater than the standardmodulator sample rate in order to detect signal peaks more accurately.The analog FM modulated signal is interpolated to that rate, while thedigital signal is passed to the process in terms of carrier bitmapdescribing the QPSK constellation. Each carrier is represented in termsof the sign bits of the real and imaginary parts of the constellationpoint. An oversampled IBOC time domain symbol can be determined byfilling in the constellation points based on the bitmap and inserting azero vector prior to the IFFT step. Standard IBOC pulse shaping can beperformed at the output. The carrier bitmap can also be used as areference in the constellation/spectrum correction step later on.

The digital signal is operated on and passed forward across iterations,but the analog signal is stored across iterations. Only on the finaliteration of the method is the analog signal added to the digital signalin a permanent manner. Provided that the signal content in the FMspectrum is minimized or shaped such as to have little impact on the FMsignal, then the distortion on the FM signal is minimal.

During peak reduction, the Root Mean Square (RMS) level of the digitalsignal is often reduced. As shown by block 300 in FIG. 3, by scaling thedigital signal back to the original RMS level, the signal power can bemaintained. In addition, the boundary region of the constellation duringthe constellation correction step is relaxed.

Optionally, each digital carrier may be equalized in both amplitude andphase using conventional equalization techniques. In this case theanalog signal must be equalized prior to passing it through the PAPRreduction method and the resultant analog and digital signal should notbe equalized in subsequent steps as this may change the peakcharacteristics.

In an embodiment, a complex random variable can be added to the standardconstellation correction prior to correction. Adding a random variablemay aid in finding a better solution across multiple iterations, sincecorrection is in opposition to the peak reduction step.

As a final step in the spectrum and constellation correction method eachIBOC symbol can be scaled so that each peak has a constant power withineach symbol and to limit variability from symbol to symbol. The scalinglevel should be chosen at the median peak level such that half thesymbols are scaled up and improve their constellation and half thesymbols are scaled down with a slight degradation in theirconstellation. This additional step should assist in the predistortionprocess as peaks now occur at a fixed rate.

Keeping the algorithm parameters adaptive allows the algorithm to beadaptively configured for different levels of peak compression based onexternal input parameters. For example, if a transmitter is temporarilyunable to produce a given peak power level, peaks may be compressedharder at a slight degradation in the signal constellation, butmaintaining the same signal power.

It will be understood that numerous modifications thereto will appear tothose skilled in the art. Accordingly, the above description andaccompanying drawings should be taken as illustrative of the inventionand not in a limiting sense. It will further be understood that it isintended to cover any variations, uses, or adaptations of the inventionfollowing, in general, the principles of the invention and includingsuch departures from the present disclosure as come within known orcustomary practice within the art to which the invention pertains and asmay be applied to the essential features herein before set forth, and asfollows in the scope of the appended claims.

INDUSTRIAL APPLICABILITY

The present method will allow existing hybrid installations to increasetheir carrier levels without needing to upgrade their entiretransmitter. In addition, the present method improves the improves theoverall efficiency of the transmitter, and will have a positive economicimpact for broadcasters.

1-21. (canceled)
 22. A method for peak-to-average power reduction in ahybrid signal comprising first and second separately modulated signals,the method comprising the steps of: obtaining a sample point of thehybrid signal; defining the sample point by way of a first vectorcorresponding to the first separately modulated signal and a secondvector corresponding to the second separately modulated signal; addingthe first vector to the second vector to obtain a hybrid vector;comparing the hybrid vector against a maximum desired peak threshold toidentify peaks in power; determining an error vector that reduces theidentified peaks in power of the hybrid vector to within the maximumdesired peak threshold; and adding the error vector to the second vectorto reduce at least one peak in power of the identified peaks in power.23. The method according to claim 22, wherein the first separatelymodulated signal is an analog signal and the second separately modulatedsignal is a digital signal.
 24. The method according to claim 23,wherein the analog signal comprises a radio modulated baseband carrierand the digital signal comprises orthogonal frequency divisionmultiplexing (OFDM).
 25. The method according to claim 22, furthercomprising a step of oversampling the hybrid vector prior to comparingthe hybrid vector against the maximum desired peak threshold.
 26. Themethod according to claim 22, wherein the error vector is added to thesecond vector in a direction extending from an origin of the secondvector in a complex plane representation thereof.
 27. The methodaccording to claim 26, wherein the error vector is based on a correctionvector comprising a correction function that applies a delta function tothe second vector.
 28. The method according to claim 27, wherein thecorrection function comprises applying a shifted sinc function to thesecond vector for peak reduction in-between discrete sample points. 29.The method according to claim 27, wherein the correction functioncomprises applying at least one complex frequency to the second vectorfor tone injection at varying frequencies.
 30. The method according toclaim 29, wherein the at least one complex frequency is orthogonal tosymbol carriers.
 31. The method according to claim 27, wherein thecorrection function comprises applying at least one windowed complexfrequency to the second vector for pulse injection at varyingfrequencies.
 32. The method according to claim 31, wherein the at leastone windowed complex frequency limits time domain impact and increasesspectral impact.
 33. The method according to claim 22, wherein thesecond vector has a length, and the error vector is added to the secondvector along the length of the second vector.
 34. The method accordingto claim 22, wherein the error vector is calculated in parallel andopposed to the first vector.
 35. The method according to claim 22,further comprising a step of correcting a constellation and a spectrumof the second modulated signal following peak determination and peakreduction to yield a corrected second modulated signal.
 36. The methodaccording to claim 22, further comprising a step of correcting aconstellation and a spectrum of the error vector.
 37. The methodaccording to claim 35, wherein the step of correcting the constellationand spectrum further comprises the steps of: demodulating the secondseparately modulated signal; correcting all carriers in the demodulatedsignal; clipping non-carriers to a mask; and modulating the correctedsecond separately modulated signal.
 38. The method according to claim37, further comprising a step of adding the first separately modulatedsignal to the corrected second separately modulated signal.
 39. Themethod according to claim 38, further comprising a step of adding acomplex random variable to the demodulated signal prior to correctingthe carriers in the demodulated signal.
 40. The method according toclaim 37, wherein the step of demodulating the second separatelymodulated signal further comprises: removing pulse shaping of the secondseparately modulated signal; and applying a Fast Fourier Transformationoperation on the carriers in the demodulated signal.
 41. The methodaccording to claim 35, wherein the step of modulating the correctedsecond separately modulated signal further comprises: applying anInverse Fast Fourier Transformation to the corrected second separatelymodulated signal to yield a transformed signal; and applying pulseshaping to the transformed signal.
 42. A computer readable memory havingrecorded thereon statements and instructions for execution by a computeradapted to carry out the method according to claim 22.